As a method for obtaining the volume fraction of cells in a cell suspension, a method using the low frequency electrical conductivity (a frequency of 100 kHz or less) of the suspension and the low frequency electrical conductivity of a solvent containing no cells (a frequency of 100 kHz or less) is known (Non-Patent Literature 1). For example, in a suspension in which spherical cells are dispersed thinly, the volume fraction of cells can be obtained in the manner of Mathematical Formula (1) below.
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                      ϕ        =                  2          ⁢                                                    κ                a                            -                              κ                b                                                                    2                ⁢                                                                  ⁢                                  κ                  a                                            +                              κ                b                                                                        (        1        )                ϕ: volume fraction    Ka: electrical conductivity of solvent    Kb: electrical conductivity of suspension
On the other hand, in the case where the shape of the cell suspended is not spherical, it is necessary to consider also the shape of the cell; for example, in a suspension in which spheroidal cells are dispersed thinly, the volume fraction of cells can be obtained in the manner of Mathematical Formula (2) below.
                              [                      Math            .                                                  ⁢            2                    ]                ⁢                                                                                      ϕ        =                  9          ⁢                                                    (                                                      1                                          1                      -                      Lz                                                        +                                      2                                          1                      -                                              L                        ⁡                                                  (                                                      x                            =                            y                                                    )                                                                                                                    )                                            -                1                                      ·                                                            κ                  a                                -                                  κ                  b                                                                              2                  ⁢                                                                          ⁢                                      κ                    a                                                  +                                  κ                  b                                                                                        (        2        )                ϕ: volume fraction    Ka: electrical conductivity of suspension    Kb: electrical conductivity of suspension    Lx, Ly, Lz: parameters related to shape etc. of spheroid
In a thick suspension that has a high density of cells and cannot be treated as a thin dispersion system, it is necessary to use another formula taking the interaction between cells into consideration.
However, in these conventional methods, it is necessary to use the electrical conductivity of a solvent containing no cells, and the volume fraction of cells cannot be found from data of the suspension alone. Furthermore, the cells need to be dispersed, not aggregated, in the solvent. Hence, for example, in the case of blood containing blood plasma components, the volume fraction etc. of erythrocyte cannot be found by conventional methods because in general rouleaux, aggregation, etc. of erythrocytes will occur. In addition, the degree of rouleaux or aggregation will change variously depending on the flowing of blood and the time from being allowed to stand; thus, it will be difficult to find a calculation formula in which the degree of rouleaux or aggregation is taken into account, and such a mathematical formula is not known at present.
A rouleau (aggregation) of erythrocytes will be formed in a way that an aggregate grows linearly in the beginning and then a spherical structure is formed (Non-Patent Literature 2). Then, the aggregate that has grown to a spherical structure will start to settle down (the sedimentation of erythrocytes (erythrocyte sedimentation)). Here, it is presumed that the larger the size of one erythrocyte aggregate is, the higher the erythrocyte sedimentation rate is. In clinical terms, since erythrocyte sedimentation is exacerbated by infectious diseases etc., it has been used as an important item of a blood test from old times to the present.
These days, a technology of simply and accurately evaluating the degree of coagulation of blood, not rouleaux (aggregation) of erythrocytes, is being developed. For example, Patent Literature 1 discloses a technology in which information on blood coagulation is acquired from the dielectric constant of blood, and describes “a blood coagulation system analyzing device including a pair of electrodes, applying means for applying an AC voltage to the pair of electrodes at prescribed time intervals, measuring means for measuring the dielectric constant of blood disposed between the pair of electrodes, and analyzing means for analyzing the degree of working of a blood coagulation system using the dielectric constant of blood measured at the time intervals after the anticoagulant effect working on the blood is removed.”